In many communication systems, for example, in systems using frequency hopping, time division multiple access (TDMA), or fast transmissions of short messages, data are transmitted in bursts. In these and similar cases, noncoherent demodulation is preferable because it allows radical reduction of synchronization time and complexity. Reduction of synchronization time and complexity increases overall energy efficiency of a communication system. Noncoherent demodulation is also advantageous when simple system implementations are required.
High overall energy efficiency in a communication system is always desirable, and it is absolutely necessary in systems using battery-powered radios. The overall energy efficiency is determined by several factors. The first factor is the amount of time it takes for the transmitter and the receiver to be synchronized. Shorter synchronization times mean more time can be spent for the transmission of data. This factor is especially important for communication systems using burst transmissions. The second factor is the level of circuit complexity needed to implement the chosen modulation/demodulation method. Simple circuit design contributes to overall energy efficiency because it reduces power consumption of a transmitter and especially a receiver. This factor is exceptionally important for short-distance communications where the power of a transmitted signal is comparable with or even lower than power consumption of the receiver. The third factor is noise immunity of a modulation/demodulation method used in the system. High noise immunity reduces the energy necessary for transmission of given amount of information with required reliability. The fourth factor is efficiency of transmitter power utilization provided by the modulation/demodulation method. High efficiency of transmitter power utilization lowers the power consumption required for given transmitted power. Other factors that influence overall energy efficiency of a communication system are not related to the modulation/demodulation methods. Therefore, they are not considered here.
Thus, there is a great need for a modulation/demodulation technique that is immune to noise and interference, efficiently uses transmitter power, and is simple to implement. Taking into account that noncoherent demodulation reduces synchronization time and complexity, such a method will maximize overall energy efficiency of a communication system because it will improve all four factors that determine this efficiency.
Differential binary phase shift keying (DBPSK) is the most noise immune binary modulation in additive white Gaussian noise (AWGN) channels that allows noncoherent demodulation [1-4]. It also allows the simplest realization of the modulators and noncoherent demodulators. Comparison of DBPSK with M-ary modulation techniques that allow noncoherent demodulation shows the following. When noncoherent demodulation is used, DBPSK is much more noise immune than any M-ary phase modulation including non-coherently demodulated differential quadrature phase shift keying (DQPSK) and differential offset quadrature phase shift keying (DOQPSK), although it has lower bandwidth efficiency. Non-coherently demodulated DBPSK is more bandwidth efficient than M-ary orthogonal keying, and the difference in bandwidth efficiency between DBPSK and M-ary orthogonal keying increases with the growth of M. At the same time, non-coherently demodulated DBPSK is more noise immune than M-ary orthogonal keying when M≦4. Only when M>4, does M-ary orthogonal keying provide higher noise immunity than non-coherently demodulated DBPSK. DBPSK allows the simplest practical realization compared to all alternative modulation techniques. It also provides the greatest ease and the highest noise immunity of bit synchronization and tracking. Additional advantage of DBPSK is the fact that the difference in noise immunity between noncoherently demodulated DBPSK and coherently demodulated BPSK is insignificant. Thus, the high noise immunity of noncoherent demodulation and synchronization as well as simplicity of realization and implementation are advantages of DBPSK.
The main drawback of DBPSK is inefficient utilization of the transmitter power because of frequency side-lobe regeneration. Indeed, the passage of a filtered conventional DBPSK signal through nonlinear circuits causes significant regeneration of the side-lobes of the signal spectrum. These side lobes can interfere with neighboring channels.
The cause for the side-lobe regeneration is as follows. A bandpass signal modulated by conventional DBPSK has a constant envelope, and its spectrum has significant side lobes. Filtering that suppresses these side lobes radically changes the signal envelope. After this filtering, the envelope starts to go up and down, and 180° phase transitions, which are inherent in conventional DBPSK and BPSK, drive the envelope to zero. Hard-limiting the signal, for example, in a class C amplifier, restores its almost constant envelope at the expense of regeneration of the signal spectrum side lobes and reduction of the noise immunity of the signal reception.
To avoid side-lobe regeneration, all units of the transmitter analog signal path including the transmitter power amplifier (PA) have to be operated in linear mode (for example, class A). However, a linear mode of the transmitter analog signal path (and, first of all, a linear mode of its PA) does not allow efficient utilization of the transmitter power due to a high peak factor of filtered DBPSK and BPSK signals. As a result, the transmitter power cannot be efficiently utilized.
Thus, despite its high noise immunity and simplicity DBPSK cannot provide sufficiently high overall energy efficiency in a communication system due to its inefficient utilization of the transmitter power.
It has to be stated that, although 180° phase transitions between adjacent symbols cause the side-lobe regeneration, the transmission of data by binary signals with opposite phases (antipodal signals) is the reason for high noise immunity of conventional BPSK and DBPSK. Reduction of the phase shift between signals from 180° to 90° radically lowers noise immunity of the modulation, on the one hand, and fundamentally reduces the side-lobe regeneration enabling better utilization of transmitter power, on the other. These competing phenomena do not allow achieving sufficiently high overall energy efficiency of a communication system with conventional BPSK and DBPSK.
Therefore, known methods [5-8] that mitigate regeneration of the signal spectrum side lobes in nonlinear amplifiers are based on providing smooth phase transition from 0° to 180° and vice versa. These methods have two significant drawbacks: (1) they do not allow noise immune noncoherent reception of DBPSK (noncoherent reception of BPSK is impossible), and (2) they cannot be implemented using only digital circuits (in other words, analog and/or mixed signal circuits are required for their implementation).
These methods are not energy efficient for two reasons. First, coherent demodulation requires a much longer synchronization time than noncoherent demodulation. Second, the impossibility of their digital realization increases power consumption and makes unfeasible their implementation as a part of a universal reconfigurable digital modem that enables significant reduction of power consumption in real world applications.